Only pass -UNDEBUG when compiling C/C++

[etcwilde]

Summary

This patch limits adding -UNDEBUG to C and C++ files so that projects can include files compiled with compilers that don't recognize this argument (Swift e.g.).

add_definitions does not expand generators, hence the change from add_definitions to add_compile_options.

[etcwilde]

Thanks for the review. Could you land this for me? I don't have commit privileges yet. Thanks.

[compnerd]

GIT 9b84dabc5f3

[mstorsjo]

This is causing build failures for me with CMake 3.10, also see https://github.com/llvm/llvm-project/commit/9b84dabc5f36f7e027c62b5c00ea97e21cfcacdd#commitcomment-37081955 for discussion about this issue.

[etcwilde]

In D73830#1853864, @mstorsjo wrote:

This is causing build failures for me with CMake 3.10, also see https://github.com/llvm/llvm-project/commit/9b84dabc5f36f7e027c62b5c00ea97e21cfcacdd#commitcomment-37081955 for discussion about this issue.

Fix posted: https://reviews.llvm.org/D73858
I don't have commit privileges yet, so someone will have to merge it for me once approved.

llvm/cmake/modules/HandleLLVMOptions.cmake

 if( LLVM_ENABLE_ASSERTIONS )
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
   if( NOT MSVC )
     add_definitions( -D_DEBUG )
   endif()
   # On non-Debug builds cmake automatically defines NDEBUG, so we
   # explicitly undefine it:
   if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
-    add_definitions( -UNDEBUG )
+    add_compile_options($<$<COMPILE_LANGUAGE:C,CXX>:-UNDEBUG>)
     # Also remove /D NDEBUG to avoid MSVC warnings about conflicting defines.
     foreach (flags_var_to_scrub
         CMAKE_CXX_FLAGS_RELEASE
         CMAKE_CXX_FLAGS_RELWITHDEBINFO
         CMAKE_CXX_FLAGS_MINSIZEREL
         CMAKE_C_FLAGS_RELEASE
         CMAKE_C_FLAGS_RELWITHDEBINFO
         CMAKE_C_FLAGS_MINSIZEREL)